1. Field of the Invention
The present invention relates to X-ray imaging apparatuses capable of capturing an X-ray image of a subject.
2. Description of the Related Art
For X-ray imaging for medical diagnostic purposes, digital imaging apparatuses in which an imaging unit including a photoelectric conversion element performs imaging have been widely used in recent years. Such a digital imaging apparatus is capable of producing an X-ray image without developing a film etc., and thus is advantageous in terms of immediacy over an imaging apparatus using a film.
In a known X-ray imaging apparatus including a photoelectric conversion element, an imaging unit and an X-ray generating unit are connected to each other via a dedicated line. This allows communication of drive timing signals and timing signals for X-ray generation, as well as power and image information, between the imaging unit and the X-ray generating unit.
In an X-ray imaging apparatus having an imaging unit including a photoelectric conversion element, after X-ray radiation, electrons converted by X-rays are read from the photoelectric conversion element. Thus, an image representing a distribution of transmittance of x-rays radiated during a specific period of time is produced.
There are broadly two different patterns of timing for reading out electrons. In one method, an X-ray generating unit transmits readout timing to an X-ray imaging unit for synchronization therebetween. Specifically, after completion of X-ray radiation, the X-ray generating unit transmits an X-ray radiation completion signal to the X-ray imaging unit. Upon receipt of the X-ray radiation completion signal, the X-ray imaging unit terminates accumulation of electric charge and starts reading out electrons.
In another method, as described in Japanese Patent Laid-Open No. 2005-46203, an X-ray generating unit radiates X-rays during a period of time in which it is permitted to radiate X-rays. Then, after receipt of an X-ray generation signal, an X-ray imaging unit reads out an image during a fixed period of image accumulation time.
In Japanese Patent Laid-Open No. 2005-46203, in response to instructions from a first switch to prepare for X-ray imaging, the X-ray generating unit and the X-ray imaging unit start preparing for imaging. Then, accumulation of electric charge is started. Here, a time delay may occur depending on the method for connection between the X-ray generating unit and the X-ray imaging unit.
Generally, a connection between an X-ray imaging unit and an X-ray generating unit can be made via a general-purpose line or a wireless connection, as well as via a dedicated line. As long as necessary functions are provided, a connection using a general-purpose line or a wireless connection is advantageous over a connection using a dedicated line, in terms of usability and cost. Examples of general-purpose lines include Ethernet (registered trademark) and Gigabit Ethernet (registered trademark). Examples of wireless connections include a wireless local area network (LAN) connection and a connection which allows communication using low and high frequency electromagnetic waves, such as those used in dedicated short-range communication.
However, when a connection method other than that using a dedicated line is used, that is, for example, when a wireless connection is used, external factors including transmissibility of electromagnetic waves may prevent the connection from being established at one time. As a result, many retries for the connection may cause a time delay. When a network line is used, line congestion may reduce communication speed and may also cause a time delay. Reasons for using a connection method other than a dedicated wired connection, that is, reasons for using a wireless connection or the like are that digital X-ray imaging apparatuses are demanded to achieve improved usability and reduced cost, as well as high-quality images.
In a known X-ray imaging apparatus having a fix period of accumulation time, there may be a period of time in which signals cannot be accumulated during X-ray radiation. That is, there may be a period of time in which unnecessary x-ray radiation takes place. In other words, the known X-ray imaging apparatus does not optimize the timing control of imaging drive signals in accordance with different conditions.
Japanese Patent Laid-Open No. 2005-46203 discloses a control method for controlling an X-ray imaging operation performed by the X-ray generating unit and the X-ray imaging unit. The X-ray generating unit generates X-rays, while the X-ray imaging unit accumulates electric charge corresponding to the amount of the X-rays and outputs an electric signal. This control method causes the X-ray generating unit to prepare for X-ray generation in accordance with an instruction to prepare for X-ray imaging. The control method includes the steps of starting accumulation of electric charge after the X-ray imaging unit completes preparation for X-ray imaging, generating X-rays in accordance with an instruction to perform X-ray imaging, and terminating the accumulation of electric charge and the generation of X-rays when a predetermined period of time has elapsed since the start of accumulation of electric charge.
However, Japanese Patent Laid-Open No. 2005-46203 does not provide a means for changing the time for terminating the accumulation of electric charge depending on the connection method, accumulation time, and information about a region whose image is to be captured.
An X-ray imaging apparatus disclosed in Japanese Patent No. 3105520 includes an image recording device and a synchronizing device. The image recording device applies a recording pulse to the synchronizing device, which adjusts synchronization of an X-ray source to changes in periodical operation.
In the technique disclosed in Japanese Patent No. 3105520, the X-ray imaging apparatus transmits an imaging drive signal for synchronization of the X-ray source. However, there is no means for changing the drive method, such as timing of the imaging drive signal, depending on the connection method etc.
Japanese Patent Laid-Open No. 2001-276032 discloses an imaging apparatus that has a configuration for preventing degradation of image quality caused by vibrations and electromagnetic noise resulting from grid movement. With this configuration, the imaging apparatus is capable of capturing high-quality images most suitable for medical diagnosis. In this imaging apparatus, after a drive operation for driving the movement of a movable grid is stopped, a readout operation for reading out an accumulation signal of an image pickup element is started.
However, in the imaging apparatus disclosed in Japanese Patent Laid-Open No. 2001-276032, the accumulation of electric charge is terminated after the moving grid is stopped. There is no means for changing the time for terminating the accumulation of electric charge depending on the connection method, accumulation time, and information about a region whose image is to be captured.
A radiation imaging apparatus disclosed in Japanese Patent Laid-Open No. 2005-312949 includes a conversion circuit unit in which a plurality of pixels including conversion elements are arranged in an array on a substrate, a readout circuit unit configured to read out a signal from the conversion circuit unit, and a radiation generation switch used to instruct a radiation generating unit to emit radiation. An imaging operation period for capturing a radiation image includes two operation periods: an idling operation period before emission of radiation, and a reading operation period starting at the time of emission of radiation. To enable the operator to operate the radiation generation switch at appropriate time immediately after the end of the idling operation period, the radiation imaging apparatus generates an emission stimulating signal to the operator during the idling operation period.
The radiation imaging apparatus disclosed in Japanese Patent Laid-Open No. 2005-312949 has no means to change the drive method, such as accumulation time, depending on the connection method etc. However, if an X-ray imaging apparatus including a photoelectric conversion element performs communication using a connection method other than a connection method using a dedicated line, synchronization with an X-ray generating unit may be lost or radiation may take place during a period of time in which image accumulation is not performed.
In an X-ray imaging apparatus that performs a reading operation during image accumulation time, the above problems may be solved by significantly increasing the image accumulation time. However, as the image accumulation time increases, dark current noise of a photoelectric conversion element increases and, as a result, image quality may be adversely affected.